The present invention relates to wireless communications, and in particular to a phased-array transmitter adapted for use in wireless communication systems.
Omni-directional communication systems have been used extensively in various applications due, in part, to their insensitivity to orientation and location. Such systems, however, have a number of drawbacks. For example, the transmitter in such systems radiates electromagnetic power in all directions, only a small fraction of which reaches the intended receiver; this results in a considerable amount of waste in the transmitted power. Thus, for a given receiver sensitivity, a relatively higher electromagnetic power needs to be radiated by an omni-directional transmitter as compared to a directional transmitter. Furthermore, because the electromagnetic propagation is carried out in all directions, the effects of phenomenon such as multi-path fading and interference are more pronounced.
In a single-directional communication system, power is only transmitted in one or more desirable directions. This is commonly achieved by using directional antennas (e.g., a parabolic dish) that provide antenna gain for some directions, and attenuations for others. Due to the passive nature of the antenna and the conservation of energy, the antenna gain and its directionality are related; a higher antenna gain corresponds to a narrower beam width and vice versa. Single-directional antennas are often used when the relative location and orientation of the transmitter and receiver are known in advance and do not change quickly or frequently. For example, this may be the case in fixed-point microwave links and satellite receivers. Additional antenna gain at the transmitter and/or receiver of such a communication system may improve the signal-to-noise-plus-interference ratio (SNIR), and thereby increase the effective channel capacity. However, a single-directional antenna is typically not well adapted for portable devices whose orientation may require fast and frequent changes via mechanical means.
Multiple antenna phased-array systems may be used to mimic a directional antenna with a bearing adapted to be electronically steered without requiring mechanical movement. Such electronic steering provides advantages associated with the antenna gain and directionality, while concurrently eliminating the need for mechanical reorientation of the antenna. Moreover, the multiple antennas disposed in phased-array systems alleviate the performance requirements for the individual active devices disposed therein, and thus make these systems more immune to individual device failure.
Multiple antenna phased-array systems (hereinafter alternatively referred to as phased-arrays) are often used in communication systems and radars, such as multiple-input-multiple-out (MIMO) diversity transceivers and synthetic aperture radars (SAR). Phased arrays enable beam and null forming in various directions. However, conventional phased-arrays require a relatively large number of microwave modules, adding to their cost and complexity.
Higher frequencies offer more bandwidth, while reducing the required antenna size and spacing. The industrial, scientific, and medical (ISM) bands at 24 GHz, 60 GHz are suited for broadband communication using multiple antenna systems, such as phased-arrays, and the 77 GHz band is suited for automotive RADARS. Furthermore, the delay spread at such high frequency bands is smaller than those of lower frequency bands, such as 2.4 GHz and 5 GHz, thus rendering such high frequency bands more effective for indoor uses, allowing higher data rates. A ruling by the FCC has opened the 22-29 GHz band for automotive radar systems, such as autonomous cruise control, in addition to the already available bands at 77 GHz.
A phased-array receiver includes a multitude of signal paths each connected to a different one of a multitude of receive antennas. The radiated signal is received at spatially-separated antenna elements (i.e., paths) at different times. A phased-array is adapted to compensate for the time difference associated with the receipt of the signals at the multitude of paths. The phased-array combines the time-compensated signals so as to enhance the reception from the desired direction(s), while concurrently rejecting emissions from other directions.
In a phased-array transmitter, each element radiates the same signal delayed by different time intervals. As shown in FIG. 1, the transmitted outputs add up coherently in the desired direction, increasing the signal power. Incoherent addition of the outputs in other directions attenuates the signal power resulting in reduced interference at receivers that are not targeted.
RF phase-shifting is unsuitable in the transmit path due to nonlinearity and variability of gain with phase-shift. Large physical size of passive components render analog phase shifting unfeasible at low frequencies. High power requirements of additional digital-to-analog converters (DACs) and high-speed digital signal processor (DSPs) preclude digital base band phase shifting